Large quantities of sulfur are produced by the CLAUS process by reacting the hydrogen sulfide originating, for example, from deacidification of natural gases or from treatment of petroleum products with sulfur dioxide formed, for example, by combustion of H.sub.2 S or combustion of sulfur or sulfur-containing compounds such as pyrites, according to the reaction: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
A substantial fraction of the sulfur produced by the CLAUS process is stored in the liquid state in heat-insulated tanks, then transported in liquid form to the places of use by trucks, boats, trains or pipelines. The transportation means is provided with adequate heat-insulating means to permit the sulfur to remain in the liquid state during the transportation.
The liquid sulfur obtained by the CLAUS process always contains, in a dissolved state, a small amount of H.sub.2 S and hydrogen polysulfides, also called sulfanes, of the formula H.sub.2 S.sub.x wherein x represents a number equal to or higher than 2. The sulfanes slowly decompose to release H.sub.2 S. The gaseous phase above the liquid sulfur in the storage tank or in the container used for its transportation contains, therefore, a certain proportion of H.sub.2 S, which, due to its toxicity and to its tendency to spontaneous inflammation, makes the loading and unloading of the containers that serve to transport the liquid sulfur dangerous. In order that these operations can be performed safely, the CLAUS liquid sulfur is generally subjected, prior to storage and transportation, to a degasification treatment whose purpose is to lower the content of free and combined H.sub.2 S of the liquid sulfur below a threshold fixed by the practice at 10 ppm.
The total amount of H.sub.2 S and of sulfanes in the CLAUS liquid sulfur is generally between 50 and 700 ppm and generally depends on the concentration of H.sub.2 S in the gaseous phase above the liquid sulfur and on the temperature of the latter. In addition, the relative proportions of free H.sub.2 S and sulfanes dissolved in the liquid sulfur depend also on the temperature of the sulfur.
The process of removal of the H.sub.2 S present in the liquid sulfur in free and combined forms includes two phases; a first phase in which the sulfanes are decomposed according to the reaction H.sub.2 S.sub.x .fwdarw.H.sub.2 S+S.sub.x-1 and a second phase in which the released and simply dissolved H.sub.2 S and the light sulfanes are removed from the liquid sulfur.
The decomposition of the sulfanes to H.sub.2 S and sulfur is a slow reaction. The rate of removal of the H.sub.2 S and sulfanes is limited by the speed of the decomposition reaction.
Most of the processes proposed for removing the free and combined H.sub.2 S dissolved in the liquid sulfur, are of the type in which a catalytic system consisting of ammonia or compounds that release ammonia at the liquid sulfur condition or compounds having a basic character in the sense of BRONSTEDT are added to the liquid sulfur. The catalytic system facilitates the decomposition of the sulfanes, the released H.sub.2 S being simply dissolved in the sulfur is separated from the liquid sulfur by any method that allows the H.sub.2 S dissolved physically to escape from the liquid sulfur or to be transformed in situ into sulfur under the action of an oxidizing gas. In particular, in the process disclosed in EP-0045636, the catalytic system consists of a compound selected from the inorganic compounds of phosphorus, urea, urea derivatives, dithionates, dithionites, thiosulfates, bisulfates, and bisulfites. In the processes described in U.S. Pat. No. 3,364,655, FR No. 2,159,691 and U.S. Pat. No. 4,131,437, ammonia is used as the catalyst and the H.sub.2 S released by decomposition of the sulfanes is removed by atomization of the liquid sulfur (U.S. Pat. No. 3,364,655), or stripping with an inert gas (FR No. 2,159,691), or by sweeping the open surface of the liquid sulfur by means of a gas such as water vapor, nitrogen, air or residual gas from a sulfur plant (U.S. Pat. No. 4,131,437). The use of a catalytic system of the type which utilizes ammonia, ammonium salts, amines and other nitrogenous compounds has also been suggested in FR No. 2,185,587 and U.S. Pat. No. 3,447,903 with transformation of the H.sub.2 S formed, in situ into sulfur by the action of an oxidizing gas namely, air in the first case and SO.sub.2 in the second case, injected into the liquid sulfur.
The above cited processes have the inconvenience of slow reaction rates which hinder carrying out the process continuously at the exit from a sulfur manufacturing process. In effect, the processes disclosed require several hours, for example, at least 2.5 hours when using a catalytic system formed of compounds such as defined in EP-0045636, or at least from 5 to 8 hours when using ammonia as the catalytic system to obtain a liquid sulfur having a content of free and combined H.sub.2 S below the level required by the standards.